Question

How do you write $y = {x^2} - 8x + 20$ into vertex form?

Answer 1

Hint: Here, we will use the completing the square method and simplify the given equation. Then we will then compare the obtained equation to the general vertex for a parabola. We will simplify it further to get the required answer. A quadratic equation is an equation that has the highest degree of 2 and has two solutions.

Formula Used:
We will use the following formulas:
1. Vertex form: $y = a{\left( {x - h} \right)^2} + k$, where $\left( {h,k} \right)$ are the coordinates of the vertex and $a$ is a multiplier.
2. ${a^2} - 2ab + {b^2} = {\left( {a - b} \right)^2}$

Complete step by step solution:
The given equation is: $y = {x^2} - 8x + 20$
Now, we know that the general equation of a parabola in vertex form is:
$y = a{\left( {x - h} \right)^2} + k$
Where $\left( {h,k} \right)$ are the coordinates of the vertex and $a$ is a multiplier.
Hence, using completing the square method and adding and subtracting the square of a constant, we can rewrite the given equation as:
$y = {\left( x \right)^2} - 2x\left( 4 \right) + {\left( 4 \right)^2} - {\left( 4 \right)^2} + 20$
Thus, using the identity ${a^2} - 2ab + {b^2} = {\left( {a - b} \right)^2}$, we get,
$\Rightarrow y = {\left( {x - 4} \right)^2} - 16 + 20$
Subtracting the like terms, we get
$ \Rightarrow y = {\left( {x - 4} \right)^2} + 4$
Thus, we can see that this equation is in the general form $y = a{\left( {x - h} \right)^2} + k$

Therefore, the given equation $y = {x^2} - 8x + 20$ in vertex form can be written as $y = {\left( {x - 4} \right)^2} + 4$
Hence, this is the required answer.

Note:
The vertex form of a quadratic is given by $y = a{\left( {x - h} \right)^2} + k$, where $\left( {h,k} \right)$ is the vertex. The "$a$" in the vertex form is the same "$a$" as in $y = a{x^2} + bx + c$ (i.e., both have exactly the same value). We convert a given equation to its vertex form by completing the square. To complete the square, we try to make the identity ${\left( {a \pm b} \right)^2} = {a^2} \pm 2ab + {b^2}$ by adding or subtracting the square of constants and hence, taking the constant on the RHS to find the required simplified equation.